Added a MANIFEST.in file to build a python source distribution package

With this, 'python setup.py sdist' creates a valid source .tar.gz that can be uploaded to pypi and then built on other machines with 'pip install eos-py'

MANIFEST.in

+include README.md LICENSE
+global-include CMakeLists.txt *.cmake
+recursive-include 3rdparty/cereal-1.1.1/include *
+recursive-include 3rdparty/eigen3-nnls/src *.h
+recursive-include 3rdparty/glm/glm *
+recursive-include 3rdparty/nanoflann/include *
+recursive-include 3rdparty/pybind11 *
+recursive-include cmake *
+recursive-include include *
+recursive-include python *